The present invention relates to an on-line consistency measurement and control apparatus having improved reliability. The subject apparatus is employed for measuring the consistency of solid-liquid mixtures, i.e., flowing mediums having solids in suspension. More particularly, the subject apparatus is employed for measuring the consistency of non-Newtonian mixtures, that is, liquids wherein the shear rate and stress relation is non-linear, as in paper stock slurries. As referred to in the subject disclosure, consistency is defined as the ratio of weight of the solids to the total weight of the solids and liquid, for a given volume of the flowing medium.
In paper stock slurries, consistency is considered to be the percentage of fibers in the water. In the operation of certain industrial processes, it is important to be able to sense small variations in the consistency of a flowing slurry and to produce a corresponding output signal for transmission to suitable control apparatus or recording apparatus. Heretofore, various devices have been proposed for this purpose. Earlier devices were responsive essentially to the viscosity of the slurry, and thus were not well suited for use with non-Newtonian slurries. One type of earlier device for making consistency measurements of paper stock slurries was based on characteristics other than viscosity. For example, there are instruments designed to respond to shear forces apparently created by deforming the stock stream, and these instruments included a plurality of finger-like projections extending in the flow stream.
Another form of device for measuring the consistency of paper pulp stock is disclosed in U.S. Pat. No. 4,062,226 which issued to Veijo Hietala on Dec. 13, 1977 and is entitled "Device for Measuring Pulp Stock Consistency". The device of U.S. Pat. No. 4,062,226 includes a carrier shaft which extends transversely relative to the direction of flow of the paper stock slurry within a conduit, and the carrier shaft carries a sensing element extending from the shaft in the downstream direction of stock flow for effectively shearing the flowing stock and for responding to engagement with the stock for producing a torque capable of being measured for indicating consistency of the stock. The device of U.S. Pat. No. 4,062,226 includes a supporting structure that supports the carrier shaft and the sensing element carried thereby for adjustable movement upon an axis which is perpendicular to the carrier shaft and which also extends transversely with respect to the direction of stock flow. The sensing element has a surface situated in a plane that is oblique with respect to the carrier shaft so that, depending upon the direction and angle of adjustment of the carrier shaft and the sensing element carried thereby with respect to a neutral position where the oblique surface extends in the direction of stock flow, the stock will be deflected by the oblique surface to a given extent in a given direction producing in this way a torque of a direction and magnitude which depends upon the velocity of stock flow. To facilitate the angular movement and deflection of the carrier shaft, the carrier shaft is sealingly connected to the device for sensing angular movement of the shaft, and the shaft extends through an enlarged, generally conical opening in the monitoring device. The paper stock slurry flowing through the conduit consists of a mixture of wood pulp, pitch, water and chemicals, and it has been found that the pulp stock slurry enters the enlarged, generally conical area in the housing through which the shaft extends, and eventually wedges tight about the shaft thereby inhibiting the free movement of the shaft and thereby disabling the operation of the consistency monitor. In addition, the pulp stock eventually contacts the seal connection between the shaft and the housing of the measuring device, which seal connection is usually a conventional "O" ring. Eventually, the chemicals within the pulp stock destroy the integrity of the "O" ring, and this can also result in the consistency monitor being disabled.
In view of the above, it is an object of the subject invention to provide a new and improved consistency monitor including an encapsulating member for maintaining the flexibility of the carrier shaft and maintaining the integrity of the consistency monitor.
It is another object of the subject invention to increase the service life and reduce the amount of maintenance required for a consistency monitor, and to increase the accuracy and reliability of the consistency monitor over a greater period of time.
A further object of said invention is to eliminate "O" ring failure in a consistency monitor.